The present invention relates to optical reflectometry, and more particularly, to a method and apparatus for measuring the thickness, group index of refraction, and front and back surface profiles of a sample of material such as a film, web or sheet.
In many industrial processes, control and understanding of film thickness, group index of refraction and surface profiles is of critical importance. For example, the manufacture of photographic film requires the generation of a uniform base or backing for the film that is free from surface defects and thickness variations. From the point of view of process control, it is advantageous to be able to measure the film base thickness surface, and group index of refraction profiles during the film generation process rather than measuring the film base in a laboratory after the film base has been manufactured. If samples are measured off-line, correction of any machinery malfunction cannot be performed until after a considerable volume of defective material has been processed. This leads to waste. For the purposes of the present discussion, the term sample includes films, sheets webs, and other material shapes that are generally flat.
Methods for simultaneously measuring the thickness and group index of refraction of films using low coherent light interferometry in an autocorrelation configuration are known to prior art. For the purposes of this discussion, an interferometer operating in an autocorrelation configuration is defined to be an interferometer having a variable differential time delay. One embodiment of an optical autocorrelator is described, for example, in chapter 5 of Statistical Optics, by Joseph W. Goodman (John Wiley and Sons, 1985, pp. 157-170). Those skilled in the art are aware of the principles of operation of an optical autocorrelator, but certain principles will be clarified here because of their relevance to this patent. In an autocorrelating interferometer wherein light is split into two different paths and then recombined and directed to a photodiode, the detected light intensity is measured as a function of a parameter. This parameter can be the differential optical path length xcex94L of the interferometer or it can be the differential time delay xcex94t of the interferometer. These parameters are related by xcex94L=ncxcex94t, where c is the speed of light in vacuum and n is the group index of the medium (usually air) of the differential optical path. The detected light intensity expressed as a function of the differential time delay is called the coherence function of the input light. Hence, a receiver which determines the time delay between light reflected from different surfaces of a film performs the same function as a receiver which determines the path delay between light reflected from different surfaces of a film. Determining the spacing between peaks in the coherence function of the reflected light is yet another way to describe the same function. For the purposes of the present discussion, the term differential time delay shall include differential path delay.
A Michelson interferometer is an example of such an interferometer operating in an autocorrelation configuration. An example of an apparatus for measuring film thickness which utilizes a Michelson interferometer is taught in U.S. Pat. No. 3,319,515 to Flournoy. In this system, the film is illuminated with a collimated light beam at an angle with respect to the surface of the film. The front and back surfaces of the film generate reflected light signals. The distance between the two reflecting surfaces is then determined by examining the peaks in the autocorrelation spectrum generated in a Michelson interferometer that receives the reflected light as its input.
U.S. Pat. No. 5,633,712 issued May 27, 1997 to Venkatesh et al., entitled xe2x80x9cMethod and Apparatus for Determining the Thickness and Index of Refraction of a Film Using Low Coherence Reflectometry and a Reference Surfaces,xe2x80x9d discloses a method and apparatus for simultaneously determining the thickness and group index of refraction of a film using low-coherence reflectometry in an autocorrelation configuration. The apparatus includes a low coherence light source that generates a probe light signal. The film is positioned between first and second reference reflectors, the first reference reflector being partially reflecting. The probe light signal is applied to the film after passing through the first reference reflector. The portion of the probe light signal leaving the film is reflected back toward the first reference reflector by the second reference reflector. The light exiting through the first reference reflector is collected to form the input to a receiver that determines the time delay between light reflected from the top and bottom surfaces of the film as well as the change in optical path length between said first and second reflectors resulting from the introduction of said film between said first and second reflectors.
The prior art methods for measuring the surface profile of a sample include the use of a profilometer, which employs a probe to physically contact the surface of the sample and generate a surface profile. Non-contact methods of surface profile measurement include optical phase shifting interferometers as described in U.S. Pat. No. 4,955,719 issued Sep. 11, 1990 to Hayes and vertical scanning interference microscopy as described in U.S. Pat. No. 5,446,547 issued Aug. 29, 1995 to Guenther et al. These traditional non-contact methods require turning the sample over and engaging in edge and corner alignment in an attempt to measure the top and bottom surface profiles of corresponding locations.
Broadly, it is the object of the present invention to provide an improved apparatus and method for simultaneously measuring the thickness, group index of refraction, and surface profiles of a sample of material such as a thin film.
It is a further object of the present invention to provide a system that does not require contact between the film and the measuring device.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
A method of simultaneously measuring the thickness, group index of refraction, and top and bottom surface profiles of a sample, includes the steps of locating the sample between a surface of a transparent optical flat and a parallel reflective surface such that the respective optical distances between any two of the surface of the optical flat, the top surface of the sample, the bottom surface of the sample, and the reflective surface are distinct and in a known relative optical distance relationship, the distance between the surface of the transparent optical flat and the parallel reflective surface being known. A low-coherent light interferometer preferably operating in an autocorrelation configuration is employed to measure the distance between the optical flat surface and the top surface of the sample, the optical thickness of the sample, and the distance between the bottom surface of the sample and the reflective surface at a plurality of locations over the sample, employing the known relative optical distance relationships. Top and bottom surface profiles and a thickness profile of the sample are generated from the measured distances and the known distance between the surface of the transparent optical flat and the parallel reflective surface. A group index of refraction profile is generated from the measured optical thickness and generated thickness profile.
One advantage of this invention is the ability to measure top surface profilometry, bottom surface profilometry, thickness profiles and group index of refraction profiles of a sample during a single set of measurements using a single non-contact probe. Both free and clamped web or film materials can be measured.
Another advantage of this invention is that it can be applied to measure birefringent materials in which case it can measure top surface profilometry, bottom surface profilometry, thickness profiles, major and minor axes group index of refraction profiles, birefringence profiles and optical retardation profiles of a sample during a single set of measurements using a single non-contact probe.
Another advantage is that the invention can be utilized as an on-line thickness, surface and group index refraction profiler of web materials during production.